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Common Stock And Preferred Stock - Additional Information (Detail) - USD ($)
 3 Months Ended 9 Months Ended 12 Months Ended
Jan. 24, 2023
Sep. 30, 2023
Sep. 30, 2022
Sep. 30, 2023
Sep. 30, 2022
Dec. 31, 2022
Class of Stock [Line Items]            
Common stock, shares outstanding   16,097,182 16,039,141 16,097,182 16,039,141 16,043,355
Common stock, shares issued   5,763 4,506 16,287 14,369  
Proceeds from Issuance of Common Stock       $ 800,000 $ 800,000  
Restricted Stock Units Granted       $ 48.03 $ 47.46  
Share based compensation expense       $ 1,400,000 $ 1,900,000  
Percentage of fully-vested restricted stock units that directors will receive in common shares when settled       70.00%    
Percentage of fully-vested restricted stock units that directors will receive in cash when settled       30.00%    
Preferred Stock   $ 200,000 $ 200,000 $ 200,000 200,000 $ 200,000
Maximum            
Class of Stock [Line Items]            
Dividend declared       $ 100,000 $ 100,000  
Restricted Stock            
Class of Stock [Line Items]            
Restricted stock vesting period       4 years    
Restricted stock non-vested   60,271 72,428 60,271 72,428  
Fair value of liabilities associated with fully vested RSUs that will be settled in cash   $ 800,000 $ 800,000 $ 800,000 $ 800,000 $ 1,000,000
Unrecognized share based compensation   $ 800,000   $ 800,000    
Share compensation recognition period       2 years 4 months 24 days    
Restricted Stock Units Granted 18,770          
Aggregate Market Value $ 1,000,000          
Forfeitures under the stock plan       0    
Cancellations under the stock plan       0    
Restricted Stock | Maximum            
Class of Stock [Line Items]            
Restricted stock available for awards   677,500   677,500    
Restricted stock that may be awarded in any one calendar year to any one participant   20,000   20,000    
Restricted Stock | Vesting Annually            
Class of Stock [Line Items]            
Restricted stock vesting percentage annually       25.00%    
Restricted Stock Units (RSUs)            
Class of Stock [Line Items]            
Restricted Stock Units Granted       0    
Restricted stock units outstanding     39,878   39,878  
Weighted-Average Stock Price     $ 39.48   $ 39.48  
Performance Shares [Member]            
Class of Stock [Line Items]            
Restricted stock non-vested   18,770   18,770    
Restricted Stock Units Granted       $ 51.83    
Share based compensation expense       $ 300,000    
Unrecognized share based compensation   $ 900,000   $ 900,000    
Share compensation recognition period       2 years 3 months 18 days    
Restricted Stock Units Granted 18,770          
Aggregate Market Value $ 1,000,000          
Forfeitures under the stock plan       0    
Cancellations under the stock plan       0    
Series 6 | Unitil Energy Systems Inc            
Class of Stock [Line Items]            
Preferred stock, outstanding   1,727 1,861 1,727 1,861 1,861
Preferred Stock   $ 200,000 $ 200,000 $ 200,000 $ 200,000 $ 200,000
Dividend rate       6.00% 6.00% 6.00%
Dividend and Distribution Reinvestment and Share Purchase Plan            
Class of Stock [Line Items]            
Proceeds from Issuance of Common Stock       $ 854,100    
Dividend and Distribution Reinvestment and Share Purchase Plan | Average            
Class of Stock [Line Items]            
Common stock price per share   $ 52.44   $ 52.44    
Dividend and Distribution Reinvestment and Share Purchase Plan | Common Stock            
Class of Stock [Line Items]            
Common stock, shares issued       16,287